This invention relates generally to ion and plasma sources, and more particularly it pertains to the neutralization of the ion beams from such sources with some or all of the electrons from hot-filament cathode-neutralizers.
Industrial ion sources are used for etching, deposition and property modification, as described by Kaufman, et al., in the brochure entitled Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1987).
Both gridded and gridless ion sources are used in these industrial applications. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Only ions are present in the region between the grids and the magnitude of the ion current accelerated is limited by space-charge effects in this region. Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, as described in U.S. Pat. No. 5,274,306xe2x80x94Kaufman, et al. These publications are incorporated herein by reference.
In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a magnetic field in the discharge region. Because the ion acceleration takes place in a quasineutral plasma, there is no space-charge limitation on the ion current that can be accelerated in this type of ion source. Because a Hall current of electrons is generated normal to both the applied magnetic field and the electric field generated therein, these ion sources have also been called Hall-current sources. The end-Hall ion source is one type of gridless ion source and is described in U.S. Pat. No. 4,862,032xe2x80x94Kaufman, et al., while the closed-drift ion source is another type of gridless ion source and is described by Zhurin, et al., in an article in Plasma Sources Science and Technology, Vol. 8, beginning on page R1. These publications are also incorporated herein by reference.
An end-Hall ion source has a discharge region with only an outside boundary, where the ions are generated and accelerated continuously over the cross section of the region enclosed by the boundary. The shape of this cross section can be circular, elongated, or some other shape as long as there is only an outer boundary to this region.
A closed-drift ion source has a discharge region with both inner and outer boundaries, where the ions are generated and accelerated only over the cross section between these two boundaries. The shape of this cross section is usually of an annular shape. It can also be of an elongated or xe2x80x9cracetrackxe2x80x9d shape, or some other shape as long as it has two separate and distinct boundariesxe2x80x94usually inner and outer boundaries.
Both gridded and gridless ion sources use electron-emitting cathodes to neutralize the ion beams that are generated, as well as to provide electrons to sustain the discharge. These electron-emitting cathodes are most often called xe2x80x9cneutralizersxe2x80x9d in publications describing gridded ion sources, and most often called xe2x80x9ccathodesxe2x80x9d in publications describing gridless ion sources. For consistency, all such electron-emitting cathodes will herein be called xe2x80x9ccathode-neutralizers.xe2x80x9d The most common cathode-neutralizers are the hot-filament, hollow-cathode, and plasma-bridge types, all of which are described in xe2x80x9cIon Beam Neutralization,xe2x80x9d anon., CSC Technical Note, Commonwealth Scientific Corporation, Alexandria, Va. (1991). This publication is also incorporated herein by reference. Because of their reliability, low cost, and simple maintenance, hot-filament cathode-neutralizers are widely used.
Because the neutralized ion beams are also quasineutral plasmas, i.e., the electron density is approximately equal to the ion density, ion sources have also been called plasma sources. It should be noted that the electrons emitted from the cathode-neutralizer do not recombine with the ions in the ion beam. Such recombination depends on three-body collisions that are negligible at the several millitorr or less background pressure in the space between the ion source and the surface struck by the ion beam. There are, however, charge-exchange collisions between energetic beam ions and background neutral atoms or molecules so that some energetic ions become energetic neutrals and some background neutrals become low-energy charge-exchange ions. The number of ions is conserved in the charge-exchange process, so that the number of ions requiring electrons to neutralize their currentxe2x80x94whether beam ions or charge-exchange ionsxe2x80x94is unchanged by the charge-exchange process.
The proper magnitude of electron emission from the cathode-neutralizer is required to reduce or eliminate electrostatic charging damage to the surfaces near or in the ion beam, particularly the surfaces of targets and deposition substrates. A prior-art method of doing this is to set the cathode-neutralizer emission in a gridded ion source at a magnitude equal to the ion beam current. This is defined as xe2x80x9ccurrent neutralization.xe2x80x9d Current neutralization is obtained in a gridless ion source by setting the cathode-neutralizer emission at a magnitude equal to the discharge current to the anode.
In practice, the two currents are set equal to each other by comparing the readings on two meters and adjusting the emission of the cathode-neutralizer until the two readings are equal. In some cases automatic controls are used to maintain the two currents at the values at which they are set. Even though set equal, the currents can still be unequal due to errors in either reading or calibrating the meters. In addition, the dynamics of control circuits frequently results in departures from current neutralization when operating conditions are changed.
A deficiency in the magnitude of the electron emission from the cathode-neutralizer results in the elevation of the potential within the ion beam until the electron and ion currents at electrically isolated surfaces reach equal magnitudes. When the potential elevation is sufficient, the electron emission from the cathode-neutralizer is augmented by the generation of micro-arcs between the ion beam and the surrounding vacuum chamber, the work piece, or other nearby hardware. These micro-arcs are of very short duration. Depending on the degree of electron emission deficiency, they may be observed with a frequency of one or less per minute up to one or more a second. These micro-arcs result either in direct damage where the micro-arc takes place or indirect damage in the form of particulates generated by the micro-arc and deposited elsewhere.
When the magnitude of the electron emission from the cathode-neutralizer exceeds the ion beam current, the excess electrons are in many cases, but not all, able to flow to the grounded vacuum enclosure or other grounded hardware within that enclosure without generating damaging micro-arcs. The fairly common situation of the ion beam being able to dissipate excess neutralizing electrons without substantial electrostatic charging, together with variations in the accuracy of current measurements, is the justification for the common practice of setting the cathode-neutralizer electron emission somewhat greater than the value required for current neutralization.
Problems have been encountered with electrostatic charging during ion beam etching, as described in an article by Olson in the EOS/ESD Symposium, 98-332 (1998). These problems have been most serious when portions of the work piece at which the ion beam is directed are electrically isolated from each other. Differential charging of these isolated portions can result in an electrical breakdown between the two portions. Such a breakdown will damage the work piece.
As described in the aforesaid article by Olson, setting the cathode-neutralizer emission current equal to or greater than the ion beam current in a gridded ion source has been somewhat effective in reducing damage due to electrostatic charging. However, as the devices being etched have used thinner and thinner films, they have become increasingly vulnerable to electrostatic charging damage. At the same time, the increasing miniaturization has resulted in increased cost per wafer. Simply avoiding micro-arcs has not been enough to avoid damage to the expensive devices being etchedxe2x80x94generically called xe2x80x9cwork piecesxe2x80x9d herein. Olson describes voltages as low as 6.4 V as being sufficient to cause damage. More recent devices can be damaged by even lower voltages.
Electrostatic charging damage has also been observed when the ion source is used for an ion-assist, or property-modification application and dielectric coatings are being deposited. When the dielectric coating covers most of the exposed conductor area in a vacuum chamber, there is no place for an excess electron emission to go without causing electrostatic charging of the coated surfaces. If the problem is severe enough, small arcs penetrate the dielectric coating to permit the excess electrons to escape. Note that these arcs are the reverse of neutralization arcs in that electrons are escaping from the ion beam, but they can also cause damage to the work pieces.
Another prior-art method to reduce damage due to electrostatic charging has been to measure the potential of the support for the work piece (often called a stage) and to control the emission from the cathode-neutralizer to minimize the potential difference between this support and ground, which is defined as the potential of the surrounding vacuum enclosure and is usually connected to earth ground. This method is described in xe2x80x9cCSC Ion Probe Kit Neutralizer,xe2x80x9d anon., CSC Application Note, Bulletin #101-75, Commonwealth Scientific Corporation, Alexandria, Va. (1991). While this method has sometimes been used successfully, it doesn""t work reliably when the ion beam strikes surfaces that are covered with electrically-insulating layers.
From a simplified theoretical viewpoint, equal magnitudes of the ion beam current and the electron current that goes to the ion beam from the cathode-neutralizer should permit one electron to arrive at the surface struck by each ion in the ion beam, resulting in no charging of surfaces struck by the ion beam. In practice, there are second-order considerations such as the electric field due to plasma sheaths and the potential variations in the ion beam due to variations in plasma density. However, this simplified approach of having equal magnitudes of electron and ion currents in the ion beam, called current neutralization, has been successfully used when the equality of currents is accurately measured and maintained. Some power-supply circuits employing hollow cathodes have been developed that provide current neutralization precisely and automatically, without the complications or operating problems of sensing, comparing; and controlling two separate currents. These circuits depend on the operating characteristics of the hollow cathode that permit it to automatically adjust to a wide range of electron emission by small variations in operating voltage. Although plasma-bridge cathode-neutralizers have not been used in similar circuits, the similar operating characteristics of hollow-cathode and plasma-bridge cathode-neutralizers would indicate that such use would be possible.
There are no equivalent circuits for hot-filament cathode-neutralizers in which current neutralization is controlled precisely and automatically, without the complications or operating problems of sensing, comparing, and controlling two separate currents. The obstacle is determining the required heater current for this type of cathode-neutralizer. While operation is conceivably possible with some large fixed value of heater current, the lifetime of the hot-filament cathode-neutralizer would be short. To obtain near-maximum lifetime, the heater current must be maintained at a value that provides a margin of electron-emission capability, and, as the hot filament wears and the need for heater current is reduced, the heater current must be continuously reduced while maintaining this margin of electron-emission capability. Here, margin means an excess of electron-emission capability above that required for neutralization. Further, this margin of electron-emission capability must be maintained without actually being able to measure the emission capability (as opposed to the actual emission) of the neutralizer-cathode.
In summary, sensitive and expensive work pieces can be damaged by electrostatic charging. Prior-art techniques have not been adequate to avoid this charging and associated damage when an ion source is used with a hot-filament cathode-neutralizer.
In light of the foregoing, it is an object of the invention to provide an ion-beam apparatus using an ion source with a hot-filament cathode-neutralizer that provides current neutralization precisely and automatically.
Another object of the present invention is to provide such an apparatus that provides current neutralization without the complications or operating problems of sensing, comparing, and controlling two separate currents.
Yet another object of the invention is to provide such an apparatus that is simple, economical, and reliable.
Still another object of the present invention is to provide such an apparatus that maximizes the hot-filament lifetime by minimizing the over-heating of the hot-filament cathode-neutralizer used to provide a margin in electron-emission capability.
In accordance with one embodiment of the present invention, the ion-beam apparatus takes the form of a gridless ion source with a hot-filament cathode-neutralizer, in which the hot filament is heated with a current from the cathode-neutralizer heater. The cathode-neutralizer is connected to the negative terminal of the discharge supply for the gridless ion source. This connection is substantially isolated from ground (the potential of the surrounding vacuum enclosure, which is usually at earth ground) and its potential is measured relative to ground. The heater current to the cathode-neutralizer is controlled by adjusting it so as to maintain this potential in a narrow operating range. This control can be manual or automatic.